A solar cell is an electronic device that converts sunlight directly into electricity. Although highly efficient solar cells have already been fabricated using single crystals of semiconductors such as Si, GaAs, and InP, it is generally accepted that large-scale terrestrial photovoltaic power generation will only be economical when low-cost, stable and highly efficient thin film polycrystalline solar cells are developed. Thin film devices require small amounts of materials which can be processed by economical mass production techniques.
Optical, electrical and mechanical characteristics of a semiconductor film determine its suitability for solar cell fabrication. Semiconductors with optical bandgap values in the range of 1.0-1.7 eV are commonly used in the absorber layers of solar cells. Materials with much wider optical bandgaps are transparent to most of the terrestrial solar spectra, and they are often employed as window layers in heterojunction and homojunction device structures. Semiconductors with direct bandgaps have high optical absorption coefficients and can be successfully used in thin film polycrystalline solar cells which typically employ 1-5 microns thick absorber layers. In addition to its optical characteristics, electronic properties of a semiconductor such as its resistivity and minority carrier diffusion length are also important in determining the ultimate conversion efficiency attainable by a solar cell fabricated using that semiconductor. Mechanical and structural properties also play a role in device fabrication. A thin film solar cell consists of various layers of different materials deposited on a substrate. Non-optimized deposition conditions, stresses in the films and possible interactions between the various layers of materials constituting the device sometimes give rise to adhesion problems and pinholes in the active region of the cell. Such defects limit the conversion efficiency of the device and reduce its processing yield.
Group I-III-VI.sub.2 semiconductors are important candidate materials for low-cost photovoltaic applications. Compounds of Cu with Ga, In, S, Te and Se are especially important for thin film solar cells since many of these compounds have optical bandgap values well within the terrestrial solar spectra. The most promising materials of this group are CuInSe.sub.2, CuGaSe.sub.2 and their alloys.
Interest in thin film CuInSe.sub.2 solar cells was raised by the successful demonstration of a high efficiency heterojunction photovoltaic device on a single crystal of this material in 1974 (Wagner et al., Applied Physics Letters, vol. 25, p. 434, 1974). Since that date many researchers have explored various deposition techniques to prepare polycrystalline thin films of CuInSe.sub.2 that are suitable for photovoltaic applications. These techniques include spray pyrolysis, compound electrodeposition, evaporation from the compound, sputtering from CuInSe.sub.2 targets, sputtering from Cu and In selenide targets, reactive sputtering, elemental coevaporation of Cu, In and Se, and the two-stage process (selenization of elemental layers containing Cu and In). Although all of these techniques have successfully yielded thin films of CuInSe.sub.2, efficient solar cells with conversion efficiencies approaching to or over 10% have only been fabricated on films prepared by the elemental co-evaporation technique and the two-stage process.
The method of depositing CuInSe.sub.2 thin films by coevaporating Cu, In and Se onto heated substrates has been pioneered by the Boeing research group. The details of the Boeing process is given in U.S. Pat. No. 4,335,266 awarded to R. A. Mickelsen and W. S. Chen ("Methods For Forming Thin-Film Heterojunction Solar Cells from I-III-VI.sub.2 Chalcopyrite Compounds, and Solar Cells Produced Thereby," June 15, 1982). This patent describes a method of fabricating high-efficiency CuInSe.sub.2 /Cd(Zn)S heterojunction solar cells by the co-evaporation technique using a CuInSe.sub.2 absorber film with graded stoichiometry, i.e. graded copper-to-indium ratio. The resistivity of CuInSe.sub.2 is a very sensitive function of its stoichiometry. A high copper-to-indium ratio in this semiconductor gives low-resistivity material with p-type conduction. A low copper-to-indium ratio, on the other hand, yields either a higher resistivity p-type layer or even a n-type film depending upon the stoichiometry. According to a published data, resistivity of evaporated CuInSe.sub.2 changes from about 0.1 ohm-cm to 10.sup.4 ohm-cm range as the copper-to-indium ratio varies from 1.1 to 0.9 (V. K. Kapur et al., "Metallization Systems for Thin Film CuInSe.sub.2 /CdS Solar Cells," Proc. 17th IEEE Photovoltaic Specialists Conference, IEEE, New York, 1984, p. 777). The solar cells reported in the above mentioned Boeing patent use a two-layer CuInSe.sub.2 structure. This is achieved by first evaporating a low-resistivity, Cu-rich CuInSe.sub.2 film on the metalized substrate, and then depositing a high-resistivity, In-rich layer on top of the low-resistivity film. The device is completed by evaporating a window layer over the In-rich CuInSe.sub.2 layer. According to Boeing researchers, having a high resistivity CuInSe.sub.2 region near the window layer of the solar cell is essential for obtaining high efficiency devices. If a single layer of low-resistivity p-type CuInSe.sub.2 is used in the device structure then the deposition of the Cd(Zn)S window layer causes the formation of Cu-nodules at the CuInSe.sub.2 /Cd(Zn)S interface and the device efficiency is reduced. If a single layer of high-resistivity CuInSe.sub.2 film is used, on the other hand, the electrical contact at the substrate/CuInSe.sub.2 interface is adversely affected. According to the Boeing patent the low-resistivity CuInSe.sub.2 layer at the ohmic contact interface provides good adhesion, low-contact resistance, a back surface field effect and large and uniform grain size, whereas the high-resistivity CuInSe.sub.2 film near the junction area avoids the formation of Cu-nodules. CuInSe.sub.2 /CdS, and CuIn(Ga)Se.sub.2 /CdS devices with over 10% efficiency have been fabricated by many research groups in different countries using the Boeing process.
As an alternative to Boeing's co-evaporation technique, researchers at Atlantic Richfield Company have used the magnetron sputtering method to deposit CuInSe.sub.2 films of varying compositions (R. B. Love and U. V. Choudary, "Method for Forming Photovoltaic Cells Employing Multinary Semiconductor Films," U.S. Pat. No. 4,465,575, Aug. 14, 1984). The proposed device structure of this patent was the same as Boeing's, i.e. a low-resistivity CuInSe.sub.2 layer was first sputter deposited on the metalized substrate, followed by a high-resistivity CuInSe.sub.2 film on which the window layer was formed. Attempts to use only one CuInSe.sub.2 layer with uniform composition gave low-efficiency devices similar to Boeing results.
The two methods of preparing CuInSe.sub.2 films described above involve reactive deposition. In other words, in these techniques Cu and In are evaporated or sputtered onto a selected substrate in the presence of Se and they react with Se during deposition. The two-stage process, however, is quite different in that it involves two separate steps: one to deposit the elemental components of the compound on a substrate and the other to react these components to form the compound. The following will review the prior art on this very promising technique.
The two-stage process involves depositing one or more of the elemental components of a compound in the form of a thin elemental layer on a substrate and then reacting these elemental components to form the desired compound. The thin layer of elements can be deposited onto the selected substrate by various techniques such as evaporation, sputtering and electroplating. The reaction can be carried out in an inert or reactive atmosphere depending upon the elemental species in the unreacted film.
Application of the two-stage process to the deposition of Group I-III-VI.sub.2 semiconductors has been reported in 1979 (S. P. Grindle and C. W. Smith, "Preparation and Properties of CuInS.sub.2 Thin Films Produced by Exposing RF Sputtered Cu-In Films to an H.sub.2 S Atmosphere," Applied Physics Letters, Vol. 35, p. 24, 1979) and in 1982 (J. J. Binsma and H. A. Van Der Linden, "Preparation of Thin CuInS.sub.2 Films via a Two-Stage Process," Thin Solid Films, vol. 97, p. 237, 1982). These works have demonstrated that single phase CuInS.sub.2 thin films could be obtained by sulfidizing metallic Cu/In stacked layers or Cu-In alloy films deposited on selected substrates by the sputtering method or by the molecular beam deposition technique.
Application of the two-stage process to the preparation of CuInSe.sub.2 films was investigated in 1984 by Chu et al. in their paper entitled "Large Grain CuInSe.sub.2 Films" (J. Electrochemical Society, Vol. 131, p. 2182, 1984). These authors obtained single phase, large grain CuInSe.sub.2 layers by selenizing evaporated or electroplated Cu/In stacked layers in H.sub.2 Se atmosphere at elevated temperatures. However, no devices were demonstrated on these films at that time.
The attractive features of the electrodeposition technique as applied to obtaining CuInSe.sub.2 films by the two-stage process have been described in U.S. Pat. No. 4,581,108, awarded to V. K. Kapur et al., and assigned to Atlantic Richfield Company ("Process of Forming a Compound Semiconductive Material," Apr. 8, 1986). In that patent, electroplating technique was used to deposit Cu/In stacked layers which were then selenized in Secontaining atmosphere to obtain CuInSe.sub.2 thin films. International Solar Electric Technology (ISET) research group has further developed this electrodeposition/selenization method and demonstrated efficient CuInSe.sub.2 /CdS solar cells using films obtained by this technique (see for example, V. K. Kapur et al., "Low Cost Thin Film Chalcopyrite Solar Cells," Proceedings of the 18th IEEE Photovoltaic Specialists Conference, IEEE, New York, 1985 p. 1429; and V. K. Kapur et al., "Low Cost Methods for Production of Semiconductor Films for CuInSe.sub.2 /CdS Solar Cells," Solar Cells, vol. 21, p. 650, 1987). A research group in Israel has also worked on the two-stage processing of CuInSe.sub.2 using electrodeposition. In that work both Cu-In alloys and Cu/In stacked layers were electrodeposited and selenized to obtain CuInSe.sub.2 films. The morphology of the selenized layers prepared by this group, however, was not suitable for solid state solar cell fabrication and no devices with appreciable efficiency values have been reported (see for example, G. Hodes et al., "Electroplated CuInS.sub.2 and CuInSe.sub.2 Layers: Preparation and Physical and Photovoltaic Characterization," Thin Solid Films, vol. 128, p. 93, 1985).
Evaporated and selenized Cu/In stacked layers have also been used for solar cell processing. D. Dimmler et al. report on a low-efficiency (4.1%) device that was fabricated on a CuInSe.sub.2 film prepared by evaporating first a Cu then an In layer on a Mo coated substrate and then selenizing the resultant stack in Se vapor (B. Dimmler et al., Proceedings of the 20th IEEE Photovoltaic Specialists Conference, IEEE, New York, 1988). The low open circuit voltage and efficiency values observed in these devices were attributed to unfavorable defect structure and secondary phases present in the films.
A U.S. patent awarded to Atlantic Richfield Company (J. H. Ermer and R. B. Love, "Method for Forming CuInSe.sub.2 Films," U.S. Pat. No. 4,798,660, Jan. 17, 1989) describes the use of the DC Magnetron sputtering technique to deposit first Cu then In layers on Mo coated substrates which are later selenized to obtain CuInSe.sub.2 films.
It is important to note that as opposed to the reactive deposition techniques described earlier in this review of prior art the two-stage process does not intentionally deposit a CuInSe.sub.2 film with graded stoichiometry. In this method either a Cu-In alloy or a Cu/In stacked layer is deposited on a metalized substrate and then the whole structure is selenized in an Secontaining atmosphere at temperatures ranging from 350.degree. C. to 650.degree. C. During this selenization step Cu and In are believed to intermix and upon reacting with Se they form the CuInSe.sub.2 compound. Most of the prior art examples of the two-stage process involve deposition of discrete layers of Cu and In onto metalized substrates. In all of these examples the Cu layer is deposited first on the ohmic contact metal and this is followed by the In layer deposition. Although there is a possibility that such a structure may yield a relatively Cu-rich region near the metalized substrate, the published data so far does not indicate any gradation of the copper-to-indium ratio through the thickness of films prepared by the two-stage process. Since the compound film formed by the two-stage process has a uniform stoichiometry through its thickness, Cu-nodule formation described in the Boeing patent cited earlier would be a problem if the CuInSe.sub.2 has a low-resistivity. But if a high-resistivity single layer of CuInSe.sub.2 were used as the absorber material of the solar cell then the contact resistance would limit the fill factor and the efficiency of the devices.
There is, therefore, a need to develop a device structure and a processing method that can yield high efficiency thin film Group I-III-VI.sub.2 solar cells without requiring the use of the two-layer structure taught by the Boeing patent. Such new approach should allow the use of a single layer of high resistivity Group I-III-VI.sub.2 semiconductor film as the photon absorber of the solar cell without any adverse effects on the device parameters. This would offer to the processing engineer the flexibility of being able to use different deposition techniques in device processing including those methods which may not easily lend themselves to the formation of a two-layer structure. The purpose of the present invention is to provide such a method.
Other problems commonly faced in attempting to use a single layer of high resistivity Group I-III-VI.sub.2 semiconductor film for solar cell processing is the poor adhesion of this film to its substrate and its poor morphology. In the case of a two-layer structure used by Boeing and Atlantic Richfield Company, the low resistivity Cu-rich film is deposited first on the metalized substrate and it provides good adhesion, good electrical contact and uniform morphology to the high-resistivity layer grown on top of it. But in the absence of this Cu-rich layer, pinholes and film peeling may become a problem. This is discussed in our paper entitled "Low Cost Methods for the Production of Semiconductor Films for CuInSe.sub.2 /CdS Solar Cells," (Solar Cells, vol. 21, p. 65, 1987) for electroplated and selenized CuInSe.sub.2 films. Similar concerns related to poor structural properties and mechanical integrity of films prepared by the two-stage process have also been raised by other groups (see for example, G. Hodes and D. Cahen, "Electrodeposition of CuInSe.sub.2 and CuInS.sub.2 Films," Solar Cells, vol. 16, p. 245, 1986; G. Hodes et al., "Electroplated CuInS.sub.2 and CuInSe.sub.2 Layers: Preparation and Physical and Photovoltaic Characterization," Thin Solid Films, vol. 128, p. 93, 1985).
From a review of the prior art it is clear that there is a need to develop new approaches to film growth methods that would yield Group I-III-VI.sub.2 semiconductor films with good morphology and mechanical integrity and good electrical characteristics so that efficient thin film solar cells can be fabricated on such films. This present invention addresses all of these issues. It provides a means of obtaining films with good morphology, good adhesion and good electrical characteristics. It avoids the formation of metallic-nodules in the solar cell junction areas by facilitating the use of high-resistivity absorber layers without any adverse effects on device parameters.